One in three Brits admit they are guilty of ageism, and that nowhere is this more prevalent than in the workplace.

Tell that to the millions of women in their 50s and 60s who are refusing to be consigned to the scrapheap, high achievers not merely clinging on to a career, but smashing the silver ceiling and commandeering top jobs.

The current crop of female TV presenters in their 50s is a very visible example of how attitudes are changing. Women used to vanish from the screen at the first sign of a laughter line.

Yet no one would contemplate pensioning off 55-year-old Fiona Bruce or Sky News presenter Kay Burley, an ageless 58.

It's a similar story in the corporate world, where a host of women in their 50s are hitting professional peaks. Among them are female FTSE 100 chief executives Emma Walmsley, 50, who leads pharmaceuticals giant GSK, and Carolyn McCall, 57, the boss of ITV.

In the City, there is 53-year-old fund manager and mother of nine Helena Morrissey. As for the arts, the Royal Ballet has a 56-year-old prima ballerina assoluta, Alessandra Ferri.

These women may be exceptional in their fields but they reflect a wider trend. Ordinary women are also pursuing careers, by choice or necessity, at ages where, in a less enlightened era, they would have been written off.

Government figures show record numbers of women over 50 in the workforce — more than five million of them, in fact. That's double the number 25 years ago and a rise of 40 per cent over the past decade.

Beneath the statistics, a profound social shift is taking place with implications for the lives of women, their families and for businesses. As Secretary of State for Work and Pensions, as well as Minister for Women and Equalities, Amber Rudd says women of all ages are breaking barriers and 'no job or industry is off limits'.

At 56 in the upper echelons of politics, she is herself an example of the phenomenon.

She cautions there is a 'long way to go' to achieve equality for older female workers, who face a new set of challenges such as combining a career with caring for elderly relatives and dealing with hot flushes in the office.

Despite the drawbacks, however, thousands of older women are now reaching the top at work in mid and later life. Kathryn Britten, 64, a managing director at consulting firm Alix Partners in London, has never found her age or gender a barrier. She lives in Surrey with her husband of 37 years, Chris, 69, a retired chartered accountant. They have a daughter and son in their 30s and two grandchildren.

Kathryn, also a qualified chartered accountant, has no plans to retire and sees herself as a pioneer for older women at work. 'I'm not sure I've reached the pinnacle of my career yet.

'I'm always thinking there is so much more I can do. Women my age didn't have role models but now we can be those role models.' She says in her line of work, as an expert witness on accountancy issues in court cases, her years of expertise are plus points for the clients who hire her.

'I'm proud of my age and the wealth of experience it enables me to bring to every situation —my clients need that,' she says.

'But that doesn't mean my job gets easier. The more experience you get, the more complex the work you may be engaged to do. I can work on commercial disputes worth hundreds of millions of pounds. That's never simple.'

Kathryn took six years out in her 30s to look after her children, keeping her hand in by marking and setting professional exam papers. 'My children have grown up to respect me as a working mother and I now spend time with them often.'

The hardest stage, she says, was when she felt like the filling in a 'guilt sandwich' — caught between young children and ailing parents. 'The time I didn't spend with my parents when they were in failing health and I was working can never be replaced. I'm lucky I was able to take time out with each of them in their last months.'

She believes employers who are flexible when staff need it 'will be well rewarded with loyalty'.

'When my mum became very frail I used to dread the calls saying she had fallen. Until I knew she was okay nothing else mattered. An employer who can be accommodating and sympathetic at such a time is so important.'

Businesswoman Claire Norwood, 55, went back to work five years ago, after taking nearly two decades out to bring up her sons, now aged 21 and 19. She lives in North London with husband Simon, 54, a director of photography. She didn't need to work for financial reasons, but did so for self-fulfilment.

She admits launching herself into the workplace at 50 was 'incredibly daunting' and initially she took a lowly role at a property firm. 'The job I got was only part-time and very junior, practically minimum wage because they couldn't afford to pay me more.

'I worked so hard I ended up having a breakdown. I was 50 years old and in pieces.' With the support of her husband and GP, however, she recovered. She then used her expertise to start a business of her own, Claire Norwood Property, transforming unloved properties into desirable homes.

'I desperately wanted to work again,' she says. 'I wanted to come home and say to my husband: 'I brought in that £10,000 or £20,000'.'

Claire — who was a shoe designer in her 20s and once made kitten heels for David Bowie — urges other women in their 50s to consider starting their own company.

'If they have taken time out to bring up children they might worry they can't go into business because they don't have a financial background. But I bring life experience and people skills to the business. I've got wisdom and maturity.

'My advice to women going back to work is to not allow yourself to feel invisible. Don't even think about your age. I love wearing Stella McCartney wedges and my fuchsia jacket. My hair might be thinner, I've got a bit of a tummy, but I'm still me — and I'm loving this new stage of my life.'

Discrimination on the basis of age is illegal, but it does still go on. And some experts believe ageism hits female employees harder and sooner than their male colleagues. Professor Lynda Gratton of London Business School says it begins as young as 40 for women but 45 for men.

At those ages, employees are often no longer considered for promotion or training. Other experts agree.

Baroness Ros Altmann, an economist and former government Business Champion for Older Workers, says: 'The general expectation is that women in their 50s are coming to the end of a career and have run out of potential. The attitude among employers in recruitment, promotions and training is that it isn't worth bothering. 'But if you are in your fifties you could have 20 years ahead of you. The sell-by dates on women tend to be earlier and the sad thing is that some women actually write themselves off when they don't need to.'

Employers have been slow to wake up to the needs of older female employees caring for elderly relatives, according to Baroness Altmann. 'The typical person caring for a mother or father is a daughter in their 50s and that piles on the pressure at work,' she says.

Another issue is that just when they hit their professional prime in their fifties, the menopause strikes.

Some women sail through. For others, it causes serious physical and psychological upheaval, but until recently was an unmentionable topic in the corporate world.

BBC presenter Louise Minchin, 50, recently revealed that the temperature in the studio has been lowered to help with her hot flushes. But Baroness Altmann says many ordinary women cannot seize control of the air-con and don't get the support they need in the workplace.

'Women suffer sleep deprivation, memory issues and hot flushes. It is a significant issue because line managers are often too young to have had experience of it themselves. It is the last taboo,' she says.

For some women, hopes of retiring early have been thwarted by hikes to the state pension age. That has risen from 60 to 65 and is due to go up to 66 for men and women by October next year. It will rise even further to 67 for both sexes.

Nessa Law, 50, had to go back to work full-time because she doesn't have enough pension savings after taking time out to have daughter Jessica, now 14. She lives with her partner Alexander, 53, and Jessica on the Isle of Wight.

Three years ago she took a full-time job as an admin officer for a university even though it involves a tiring commute. 'This is not what I thought I'd be doing in my 50s. It's only a junior role because I took so much time out of work so I had to start at the bottom again.

'It involves leaving the house at 7.30am, taking the boat and bus to the mainland then not getting home until nearly 7pm. While it's lovely to have a bit of extra money I'm doing this purely to get my pension.'

Whatever women's motivation for working in their 50s and 60s, and regardless of the obstacles it can present in some cases, Ros Altmann says the trend will continue.

'Not every woman in her 50s wants to work but many need to, whether because they are divorced, single or widowed, or because their pension is not enough.

'There is a lack of skills in many areas and they can fill the gaps. This is the next frontier for women.

'If society doesn't accept older women in the workplace or expects them to accept a second rate career, we are wasting the skills of a large part of the population.'